Electron-discharge device and method of operating the same



Nov. 8, 1927. 1,648,458

G. M. J. MACKAY ET AL ELECTRON DISCHARGE DEVICE AND METHOD OF OPERATINGTHE SAME Filed Aug. 27. 1926 2 Sheets-Sheet l (PIES/UM Inventors: GeorgeM.J.MacKa9, Ernest E.Char-lton,

TheLnAttohneg Nov. 8, 1927. 1,648,458

G. M. J. MACKAY ET Al.

ELECTRON DISCHARGE DEVICE AND METHOD OF OPERATING THE SAME Filed Aug.2'7. 1926 2 Sheets-Sheet 2 Q Q O \J ptfi/TEES A15LV/N 0 3, v? -2 9 $4Inventors:

"2- 2. 4 George M.J.Mackag. Q Ernest EChaPJton. 5 A 3*! DB l [06,Bvzssarr: BAR:

Patented Nov. 8, 1927.

UNITED STATES PATENT OFFICE.

GEORGE E. J. MAOKAY AND ERNEST E. CHARLTON, OF BCHENECTADY, NEW YORK,AS- SIGNOBB 'I'O GENERAL ELECTRIC COMPANY, A. CORI'OBATION 0! NEW YORK.

ELECTRON-DISCHARGE DEVICE AND METHOD 0] OPERATING THE SAME.

Application fled August 27, 1928. Serial No. 182,012.

Our present invention relates to electron discharge devices which areprovided with one or more electrodes from which electrons are emitted bya thermionic effect, that is, by reason of the temperature of theemitting surface as distinguished from external forces, such asphoto-electric action. As a consequence of our invention we have rovideddevices of the thermionic type w ich are operable at a materially higherefficiency than heretofore.

The electron emission from a thermionic cathode depends on severalfactors, the main controlling factors being the temperature at which thecathode is operatedand the nature of the cathode material. required toseparate electrons from a thermionic cathode differs with the nature ofthe emitting surface. Each material has a characteristic electronaffinity. The energy required to obtain a given electron emission from agiven material is dependent on the work function of the electronemitting material. This work function is a measure of the electronaflinity, that is, the attraction which the electron emitting surfacepossesses for the electrons.

As explained by various investigators, for example by Langmuir in theTransactions of the American Electrochemical Society, Vol. XXIX, 1916,page 125, there is an absorption of energy when electrons are emittedfrom heated metals, which is measurable as heat absorbed and which maybe calculated in terms of a potential difference in volts, as aquantitative measure of work done in separating an electron from anemitting surface. It is this value which is called the work function ofthe electron-emitting material.

In the thoriated type of thermionic cathode the electron emissivity ofthe foundation material (ordinarily tungsten) is increased by forming onthe surface of the cathode an adsorbed film of thorium. Because thethorium film has a lower work function than tungsten, a higher electronemission can be obtained at a given temperature from the thorium filmthan from tungsten. The present invention constitutes a further advanceover the thoriated type of cathodes and results in a still higherefficiency of electron production.

In accordance with our invention the elec- The energytron emissivity ofthermionic cathodes is increased by a vapor of a substance which has alower work function than the material constituting the cathode. We havediscovered that vapors of low ionization potential, in particular thevapors of certain alkali metals, are capable of forming from the vaporphase upon the surface of a thermiomc cathode an adsorbed film of lowwork function and, therefore, of high electron emissivity. In otherwords, in accordance withour invention, the effective work function ofthe cathode is substantially lowered to a value less than thatcharacteristic of the foundation material of the cathode by presence ofthe vapor.

In the practice of our invention a useful electron emission may beobtained at such low temperatures that in some cases cathodematerials,-such as nickel, for example,- may be used which even at theirmelting points would not be capable in the absence of these vapors ofgiving a useful emission of electrons. Molybdenum, tantalum and platinummay be used. We prefer, however, ordinarily to employ tungsten as thecathode .foundation material, and preferably use caesium as the materialfunctioning from the vapor phase to increase the electron emissivity.

The electron affinity (or work function) of tungsten is 4.52 volts,whereas the ionization potential of caesium vapor is about 3.9 volts. Ifa caesium atom comes near a tungsten surface, the tungsten, having ahigher electron affinity than the caesium atom, robs the caesium atom ofan electron and leaves it in the form of a positive ion. These caesiumions when close to the tungsten surface induce a negative charge on thetungsten surface and are, therefore, held to the tungsten surface byelectrostatic force. It is this force which causes the formation of theadsorbed film of caesium. A more detailed discussion of the subject maybe found in the Physical Review, Vol. 24 page 510 (1924) and theProceedings of the Royal Society A, Vol. 107, page 61, (1925).

As the rate of evaporation of such a film is of a lower order ofmagnitude than the rate of evaporation of an alkali metal in bulk, sucha film will function at suflicientl high cathode temperatures to obtainan ad vantageous electron emission from the film.

Ill

Under some conditions rubidium can be used for practical purposes inplace of caesium for obtaining the benefits of our invention.

As a consequence of our invention electron discharge devices may beoperated for various technical purposes at cathode temperatures so muchlower than heretofore reuired for the same purposes that the useful lifeof the devices may be greatly increased; also by reason of the decreasedheating energy required, the cost of electron production is reducedmaterially. 1

Adsorption of a film of alkali metal does not occur on thoriatedtungsten to an efl'ective extent, the work function of thorium beinlower than that of the alkali metals. The nefits of alkali metal ofenhancing the electron afiinity of a cathode of the thoriated type canbe obtained only under such conditions that such a cathode fails to forma complete thorium film on its surface.

The formation and maintenance of adsorbed films of otherwise vaporousmaterials may occur under two distinct sets of conditions, which, forthe sake of simplicity, may be classified as pressure conditions andvacuum conditions.

Pressure conditions.

The high electron emission characteristic of our invention may beobtained by heating a suitable thermionic cathode to a chosen electronemitting temperature in the presence of a suitable alkali metal vapor ata sufficiently high vapor pressure. The high electron emission underthese relatively high vapor pressure conditions is due mainly to thelarge number of atoms of the alkali metal striking and condensing on theheated cathode.

Vammm conditions.

The high electron emission characteristic of our invention can beobtained at vapor pressures of alkali metal too low to permit ofappreciable ionization by coll sion, that 15, at vapor pressurescorresponding to bulb temperatures below about 65 U.

For example, an emission of electrons 1S obtainable from an ordinarytungsten filament operating at a temperature somewhat below 800 K. (527C. 1n the presence of caesium vapor correspon in to a bulb temperatureof about 60 (3., which is substantially equal to the electron emissionfrom such a filament operatin in a vacuum at about 2,000 K. (1727 (E).The low pressure type of thermionic devices embodying our invention andmethods of their manufacture are covered by a copending application ofKenneth Kingdon and Irving Langmuir, Serial No. 673; 165, filed November6, 1923.

For every vapor pressure of alkali vapor, the electron emission risesfrom a minimum cathode temperature to an optimum cathode temperature andthen decreases with further rise of cathode temperature, the adsorbedfilm being driven ofl. As at lower vapor pressures the maximum emissionoccurs at ower cathode tcm eratures, the adsorbed film becomes lessective as the vapor pressure decreases. However, the emission is greatereven at low vapor pressures than the characteristic emission from anuncouted cathode at the same temperature, as will be hereinafter morefully explained in connectlon with Fig. 8.

For commercial or other uses, however. requiring substantial current,the electron emlsslon under conditions of low va or pressure of thealkali metal here caller vacuum condltions, may be and preferably isenhanced by forming upon the cathode a layer of a material having theproperty of tenaclously holding the alkali metal film upon the cathode.Electronegative substances, and in particular oxygen, increase theelectron affinity of the cathode and hold the alkali metal upon theheated cathode more tenaciously.

Methods of obtaining the benefits of the high electron emission fromalkali metals most advantageously under vacuum, or low pressureconditions by the aid of such ma terial are, however, not of ourinvention, but are dcscrlbed and claimed in said copcndingapphcatmnfiled by Kingdom and Langmum on November 6, 1923, Serial No. 673,165.

Our invention which is of a broader and more comprehensive nature, willbe described in connection with the accompanymg drawings which show inFigs. 1, 2, and 5 different modifications of thermionic re ctiryindevices embodying our invention; Fig. 3 shows a reduction tube fromwhich a charge of alkali metal may be introduced; Figs. 6, 7 andillustrate three-electrode dcvlces embodying our invention; and Figs. 8and 9 are curves indicating the relation of electron emission to suchother factors as cathode temperature and vapor pressure of activatingmaterial.

\Ve will explain our invention by describing first the fabrication of adevice in which the pressure of the va or of the activating orfilm-forming material is relatively substantial. corresponding forexample: to a bulb temperature of 150 C. and second the fabrication of adevice in which the vapor of the activating material is low, as is thecase in a device operated at or only slightly above room temperature,say at a ulb temperature of to C.

The device shown in Fig. 1 comprises a sealed envelope 1, consisting ofglass, quartz or other suitable material, and containing afilamentaryrhcathode 2 and a c lindrical anode 3. e cathode convenient yis suploo ported by leading-in conductors 4, 5 which are sealed into theenvelo e at opposite ends, a light tungsten spring being rovided tomaintain the filament taut. T e cathode ma consist of tungsten,molybdenum, nic e1 or other suitable refractory material. The anode 3also may consist of tungsten, molybdenum, nickel, or other suitableconductive material, and has because of its size a sufliciently highheat dissipating capacity to operate at a temperature too low forelectron emission. It is supported by a stiff tungsten wire 7 sealedinto the'envelope and serving also as a current supply lead The envelope1 also contains a quantity of caesium as indicated at 8, but otherwisepreferably is evacuated.

We have referred inthe following description and in the accompanyingclaims specifically to caesium but wish it to be understood thatrubidium may be similarly used and therefore, constitutes an equivalentfor ciesium.

Caesium preferably is introduced in the following manner:

The container 1 first is connected by fusion to a reduction tube, suchas shown in Fig. 3, adapted to introduce caesium preferably by reductionfrom a compound. The two containers are baked out at a temperature ashigh as the glass will permit, say about 300 0., while ases and vaporsare exhausted. The cham er 10 then is opened and a mixture of a caesiumcompound, such as the chloride, and a reducing agent, calcium, forexample, is introduced. The chamber 10 then is a ain sealed, and the twocontainers are again eated with the vacuum pump (not shown) in operationuntil the chargeand the containers are moisture-free. Thereaftersuflicient heat is applied to the chamber 10 to cause the reduction ofthe cwsium compound and the distillation of metallic caesium fromchamber 10 successively into the chambers 11 and 12, and from thenceinto the main container.

Two separate 0 erations for evacuating and introducing t e alkali metalare not necessary. As described in connection with Figs. 6 and 7 thepreparation of the devices may occur by a modified process whereby thealkali metal is released by reduction within the evacuated container.Suiiicient caesium should be introduced so that an excess of unvaporizedcaesium will be present in the container 1 at the operating ternerature. The caesium reduction tube is fina ly sealed from the mainreceptacle by fusion in the usual manner, as indicated at 9.

The method of introducin caesium is described in a copending appication, Serial No. 97,?17, filed March 26, 1926, by Ernest E. Charlton.

When the operating temperature is main tained above room temperature thecontainer 1 is surrounded by a. suitable heat. conserving jacket or aheater indicated by dotted lines 18. i

Fig. 1 shows the cathode and anode connected by conductors 14 and 17 tothe secondar of a transformer 15 in series with a load evice,represented for illustrative purposes by a storage battery 16. The.cathode 2 has been shown as heated b connection across a section 19 ofthe secon ary winding by the conductors 13 and 17. A given electronemission may be obtained under these conditions with an ordinarytungsten cathode operating at a substantially lower temperature thanwould be required to produce the same emission in the absence ofcaesium.

For the rectification of alternating current of substantial value, fortechnical pur uses, as for example, to charge a storage bfitery, weprefer to operate the device at a temperature high enough to raise thepressure of the caesium vapor to a point at which the pressure of thecaesium vapor is relatively substantial. For example at a bulbtemperature of about 150 C. the caesium vapor will have a pressure ofabout 0.02 mm. of mercury and will be ionized by the passage of currenttherethrough. In some casesit is preferable to first operate the bulb ata igher temperature, say aging, to gradually reduce the temperature to150 C. The voltage impressed u on the tube by the transformer section 19epends on conditions, particularly on the size of the battery but shouldbe high enough to produce an ionization discharge in the vapor.

The positive ions produced under these conditions bombard the cathodeand thereby cause suflicient electron emission so that external heatingmeans for the cathode is unnecessary. The switch 20 in the heatingcircuit, therefore, may be opened.

A system such as shown in Fig. 1, will select only one set of half wavesof the alternating current. If desired, a plurality of anodes may beprovided, as shown in Fig. 2 in order to secure complete rectification.The device shown in this figure has two cylindrical anodes 28 and 29 tobe connected in the usual well-understood manner to a source ofalternating current supply.

We have shown in Fig. 4 a device embodying our invention in which thecathode 23 is constituted by a small cylinder of molybdenum, tungsten ornickel connected to a sealedin conductor 24, and heated to the operatingtemperature by a resistance heater 25, which is adapted to be heated bcurrent supplied by the conductors 26. 27. The anode 3' consists of acylinder of tungsten, molybdenum, nickel or other suitable material. Itis supported by the sealed-in at 200 C. and upon conductor 7'. Thecontainer 1' is suitably evacuated and provided with a quantity ofcaesium or equivalent material.

-as shown in Fig. 1 ma A device for rectifying alternating current (tocharge a storage battery for example), be provided with a tungstencathode consisting of a filament about two inches long and four onethousandths of an inch in diameter. The anode may consist of a metalcylinder of nickel, or other suitable material. The cathode is operatedat a temperature of about 1400 to 1700 K. (1127 to 1427 0.), which iswell below white incandescence, the usual operating temperature of atungsten cathode. The bulb is heated to a temperature of 150 C. byenclosin it in a heat insulating envelope and uti izing the heatdissipated by the filament or using an external heating unit. When thetube is connected in a circuit with an alternating current power supplyas shown in Fig. 1 with sufficient alternating current voltage impressedto satisfy the constants of the circuit (for example, an alternatingcurrent voltage varying from 5 to 100 or more), a rectified current ofone half to one ampere can be obtained with a direct current potentialdrop in the tube between cathode and anode of five volts or less.

If the bulb temperature is raised to 200 C. and the cathode ismaintained at a temperature of 2000 K. a voltage drop in the tube of oneto two volts and in some cases one tenth of one volt has been obtained.

An electron emission from the cathode of the order of 3.5 amperes persq. cm. of cathode surface can be obtained.

\Ve have found that a magnetic field applied so as to increase the len hof path of the electrons enables us to oitain a given currentunder thesame conditions at a materially lower vapor pressure of caesium or thelike. In the devices shown in Fig. 5 the discharge tube 30 is surroundedby a magnetic winding 31 which generates a magnetic field substantiallyparallel to the cathode 32. The electrons passing to the anode 33 aredeflected in spiral paths about the cathode. We have obtained in adevice havin the construction shown in Fig. 5, where y a magnetic fieldwas generated, a substantial current at lower bulb temperatures thancould be obtained without a magnetic field.

We will now explain our invention in connection with devices operatingunder low pressure conditions.

We have shown for illustration in Figs. 6 and 7 a low pressure or vacuumdevice operating without substantial gas ionization and embodying theprinciples of our invention in a preferred form. This device in ts morespecific structural aspects, embodies also the invention of Kenneth H.Kmgdon described and claimed in a copending application filed on March8, 1926, Serial No. 92,946. Another form of low pressure device isdescribed later in connection with Fig. 10.

As shown in both Figs. 6 and 7 the device comprises a sealed envelope 34on the stem 35 of which are mounted the e ectrodes and other metal partsof the device. The cathode in this type of device is constituted by astraight or rectilinear filament 36 (preferably consisting of a closelywound, small diameter spiral) which is supported by an anchor 37. Thelower end of the anchor is sealed in the stem 35, the anchor extendingto the upper part of the bulb where it is connected to the upper end ofthe cathode filamcnt 3.6. The cathode 36 referabl conslsts of tungstenand, as wil be hereinafter described, this tungsten cathode preferablyshould be provided with an oxygenous or oxygen-treated surface so as toprovide a foundation in which an adsorbed filament of caesium, or othermaterial of high electron emissivity, is formed more readily than uponan ordinary tungsten surface. The anode 38 which is cylindrical in formand preferably consists of nickel, is provided upon its interior with anumber of vanes 39 extending radially toward the cathode filament. Thecylipder is supported by stout anchor wires 40,41 which are fusionsealed into the stem 35. Both the cathode leads 42, 43 and the anodelead 41 are conducted through the stem 35 in the usual manner and arelead to external contacts 44 of a standard base 45. A control electrodeor grld which also may consist of nickel is constituted by the vanes 47,48 which are affixed upon a plate 49 which conveniently is made circularin form and is positioned closely ad acent but out of contact with thelower end of the cylindrical anode 38. The plate 43 is supported uponthe stem 35 by anchor wires 50, 51 and is provided with a hole throughwhich the cathode lead passes. It serves to shield the stem 35 and thesurrounding space from the cathode so as to reduce ionization andelectron bombardment, thereby the formation of conducting deposits onthe stem.

In the preparation of the described de vice the bulb is thoroughlyevacuated after the electrodes have been mounted in position. The bulbwhen constituted of glass, preferably is heated during evacuation to atemperature of about 300 C. The filament preferably is heated to atemperature of about 2000 K. to free its surfacejrom impurities and toremove occluded gas. During evacuation which may be carried out by amercury pump, water vapor may be adsorbed by a quantity of phosphoruspentoxide contained within a branch of the exhaust system (not shown).The use of this material is accompanied by better results than the useof liquid air in a trap in the evacuating system.

After the bulb has been evacuated, about 100 microns of oxygen areadmitted to the container, the cathode filament then being heated to atemperature of about 2000 K, for about one minute in order to cause theformation of a thin layer of adsorbed oxygen upon the cathode. Theuncombmed oxygen then is pum ed out and a quantity of activating materiapreferabl caesium or rubidium is admitted into t e container. Thismaterial conveniently may be evolved by chemical reduction of acorresponding compound of the desired metal contained in a capsule 52which is afiixcd by wire 53 to any convenient support within the device,such as the anode 38. For example, the capsule 52 may contain a mixtureof caesium chloride and finely-divided calcium and may be brought to areaction temperature by the application, in the usual manner, of a highfrequency magnetic field.

It is not essential that the device should be evacuated as completely aspossible and then pure oxygen separately admitted in order to provide athin layer of adsorbed oxygen upon the cathode. It is practicable toproduce a satisfactory oxygenous surface condition upon the cathode byleaving in the bulb several hundred microns of air and then heatingthepathode in this residual air in order to form the desired foundationsurface for the film of caesium or the like. It is also desirable tohave present in the bulb a source of oxygen, such as a small piece ofoxidized copper 54. During the o ration of the tube a reaction occurswhic forms caesium oxide and the resulting mixture of caesium andcaesium oxide eliminates deleterious gases such as hydrogen and carbonmonoxide when evolved in minute quantities from the metal and glassparts of the device. The three-electrode device here described is ofparticular utility as amplifier of weak currents, such as occurring inradio circuits.

When a vacuum device prepared as above described is operated after theintroduction of caesium, or the like, with the cathode at a tem eratureof about 900 K. the bulb as a who e being at a temperature of about 30C. (303 K.) "an electron emission is obtained from the cathode of theorder of 300 milliamperes er sq. c. m. This emission is of the same 0 erof magnitude as the emission from a tungsten filament when operating inthe vacuum in the absence of an activating substance at a temperature ofabout 2500 K.

It is not essential, however, in order to secure benefits of ourinvention at low vapor pressures to pro-treat the cathode with anelectronegative gas, such as oxygen, as embodied in the devices ofFigs.6 and 7 which include also improvements covered by the otherapplications referred to herein. A low ressure device in which thecathode has not men treated is shown in Fig. 10, this device beingadapted to operate at enhanced eificicncy as an amplilicr or radiodetector.

This device comprises an evacuated recc tncle 56, a spiral filamentarytungsten ca ode 57 adapted to be heated by current supplied through theleading-in conductors 58 59, and anode 60, consisting of czesium and agrid electrode 61, consisting of a spiral conductor surrounding thecathode. Conneclions to the anode and grid may be made through theleading-in conductors 62 and 63. The container is first exhausted andthen a small quantity of the desired metal, (preferably caesium) isintroduced into the receptaclc before sealing. The caesium or otheralkali metal may be generated in a side chamber (not shown) containing amixture of caesium chloride and calcium or magnesium turnings or powder.

During operation the bulb 56 preferably should be maintained at apressure of .02 micron, corresponding to a bulb temperature of about 50to C. By proper roportioning of the device, the heat supp ied by thecathode may be made sufiicient to heat the device as a whole to thedesired 0 erating temperature. The cathode should Es operaled at atempearture of about 800 K. At temperatures materially above 1000 K. theelectron .emission decreases. The electron emission falls to a very lowvalue at about 1100 to 1200 K. When rubidium is used instead of caesiumin the bulb a somewhat higher bulb temperature should be used.

The energy required to heat a cathode in a tube embodying our inventionis very much less than the heating ener required for a device unprovidedwith activating vapor but otherwise similar. It is less than the energyrequired to obtain the same emission from a tungsten filament of thethoriated type.

The relations between the electron emission from a tungsten filamentwhich has not been treated" with oxygen in the presence of caesium, theemission from an oxygen treated tungsten filament in the presence ofcaesium are compared with electron emission from tungsten by curves inFig. 8. It should be understood that when reference is made herein to atungsten filament (without other gualification) it is intended todesignate a lament giving the characteristic emission of unalloyedtungsten, as distinguished from film forming electrodes, such asthoriated filaments, or electrodes coated with metallic oxides. In Fi 8the ordinates represent logarithms of t e current values and theabscissae represent absolute temperatures (Kelvin scale). The currentvalues are plotted as logarithms because the difierences in the valuesof the currents from the treated and untreated filaments are of adifl'erent order of magnitude and could not otherwise be plottedtogether on the same scale. The curve 65 shows the characteristicelectron emission from a tungsten filament at temperatures ranging fromabout 1500 to 2500 K. The curve 66 shows the electron emission atdill'ereut temperature from a tungsten filament in the presence ofcaesium vapor in a bulb heated to 60 C. for filament temperatures withinthe limits of about 600 to 1600 K. It will be observed that the emissionfrom such a filament rises to a maximum at a filament temperature ofabout 700 to 800 K. when the electron emission is about equal to theemission characteristic of tungsten alone at about 2000 K. The emissionthen decreases with a rise of temperature until it becomes the same asthe electron emission from tungsten in a vacuum. At cathode temperatureof about 1650" K., caesium vapor at a pressure corresponding to 60 C. nolonger has an appreciable efl'cct upon the electron emission oftungsten.

At pressures of caesium vapor corresponding to temperatures less than 60C., the enhancement of electron emission from a tungsten filament due tothe adsorption of caisium is less. but is still observable. The curve 67shows the electron emission from tungsten. in the presence of caesiumvapor at a bulb temperature of about 20 C. The maximum emission isobtained at a lower cathode temperature. that is, between 600 to 700 K.At this cathode temperature the electron emission becomes nearly equalto the emission from tungsten at about 1850 K. in the absence ofactivating vapors, namely about fifty to sixty microamperes per sq. c.m. of emitting surface.

When the tungsten previously has been provided with an adsorbed film ofoxygen, the efleetiveness of the caesium is increased enormously. Thecurve 68 shows the electron emission from an oxygen-treated tungstenfilament which rises to a maximum at about 900 K. when it is severalhundred milliamperes per square centimeter of cathode surface, which iscomparable to the characteristic emission from tungsten at 2450 K. Asalready stated above. the improvement in adsorption and electronemission due to the electronegative foundation on the cathode is theinvention of Langmuir and Kingdom, but is here set forth as a preferableembodiment of our broad invention.

Fig. 9 shows the relation of the electron emission of a heated tungstensurface to the pressure of the activating vapor (in this case caesiumvapor) the ordinates represent ing logarithms of maximum electron emission measured in amperes and the abscissce representing logarithms ofvapor pressure of caesium measured in bars 3} ar equals 0.00075 mm). Arange of ulb tempera- The curve 69 shows that the maximum emission risescontinuously with increaseof vapor pressure. At vapor pressurescorresponding to temperatures above about to C. the electron emissionwithout an electronegative foundation layer becomes substantial, beingabout one mi'lliampere per square centimeter (in the case of caesium)using an untreated tungsten filament. At vapor pressures correspondingto bulb-temperatures of about 150 to 160 C. the electron emissionamounts to several hundred milliamperes per square centimeter, a valuewhich is of the same order of magnitude as the emission at 20 C. bulbtemperature in the case of an oxygenated tungsten cathode. This factwill explain why in a device operating at relatively high vaporpressure, such as the rectifier shown in Fig. 1, it is unnecessary toprovide the cathode with an electronegative film in order to secure ahigh electron emission.

At the high vapor pressure corresponding to a temperature of 150 C. alarge amount of positive ionization occurs in the discharge space. Whena device such as shown in 1 is operated with an impressed voltage atleast as high as about two volts the discharge assumes an arc-likecharacter, and is accompanied by luminosity. At these high vaporpressures the number of atoms of caesium, or other activating vapor,striking the cathode is greater than under vacuum conditions and henceat a sufliciently high vapor pressure, the characteristic emissivity ofan adsorbed film of the material constituting the vapor appears even inthe absence of an electronegatlve film.

Although the distinction between the types of thermionic devicesembodying our invention has been pointed out in detail above, we

desire to point out as a recapitulation that these devices may begrouped as:

(a) Devices operating at bulb temperatures high enough to obtainionization in the vapor may be grouped together as pressure devicescontaining high efiicienc thermionic film-coated cathodes un rovi edwith an electronegative foundation l a er.

(2)) Devices operating at bub temperatures too low to obtain appreciableionization by collision in the vapor may be ouped as vacuum devices.This group includes devices in which film formation and hence electronemission is enhanced by treatment of the cathode with oxygen or otherelectronegative material, but of course, it is not restricted to suchdevices.

Devices of the first group form the subject matter of our priorapplication, Serial No. 604,077, filed November 29, 1922, of which thepresent application is in part a continua-i tion; devices of the secondgroup are described and claimed in Langmuir and Kingdon applicationSerial No. 673,165 of November 6, 1923.

What we claim as new, and desire to secure by Letters Patent of theUnited States,

1. An electron discharge device containing electrodes including athermionic cathode, and means for maintaining in the space surroundingsaid cathode the vapor of a ma-, terial having a lower electron afiinrt:than said thermionic ca-hode, said vapor aving during normal operationof the device a pressure capable at the operatmg temperature ofthecathode of causing 881d cathode to have a substantially higherelectron emission than said cathode would have in the absence of said vaor.

9. An electron ischarge device comprising a thermionic cathode, ananode, an enclosing evacuated container, a charge there in of alkalimetal havin a lower electron aflinity than said thermionic cathode, andmeans for maintaining said container during operation at a temperatureat which pressure of said metal is sutiicient to cause an electron-emission to be obtained from said cathode for a range of cathodetemperatures which is of a higher order of magnitude than the emissionobtainable for said range of cathode temperatures in the absence of saidvapor.

3. An electron discharge device comprising a thermionic cathode, ananode, and n'ieans for surrounding said cathode with vapor of materialhaving a lower electron affinity than said cathode and having theproperty during normal operation of the device of coacting with saidcathode to cause a substantial electron emission at a cathode ternerature at which the electron emission in t e absence of said vapor isnegligibly low.

4. An electron discharge device comprising a container, electrodestherein which include a thermionic cathode. means for heating saidcathode, a material in said container the vapor of which is capable ofimparting to said cathode a substantial electron emissivity at a cathodetemperature at which the characteristic electron emission of saidcathode is negligible, and means for maintaining said ontainer at atemperature above about 5. An electron discharge device comprising athermionic cathode, an anode, an enclosing evacuated container and acharge of caesium in said container, the surface material of saidcathode having a work function so related to the ionizing potential ofcaesium vapor that said cathode has a materially higher electronemission in the presence of said vapor than in its absence over a rangeof cathode temperatures below about 2000 K.

6. An electron discharge device comprising a sealed evacuatedreceptacle, electrodes therein including a thermionic cathod means forheating the cathode, a charge 0 alkali metal in said receptacle having alower electron allinity than said cathode, and heat-conserving means fornmintaining the vapor pressure of said alkali metal sulficiently high tocause a given electron emission to be obtained from said cathode at amaterially lower cathode temperature than would be required for the sameemission in the absence of said alkali metal.

7. An electron discharge device comprising a cathode adapted to o crateat an ele vated temperature, means or heating said cathode, an anode, anenclosin sealed receptacle,-and means for provi ing caesium vapor to thes ace within said receptacle at a pressure su ciently high to raise theelectron emissivity of said cathode to value ma terially higher than thecharacteristic emissivity of said cathode in the absence of said vapor.

8. An electrical discharge device comprising an evacuated container, athermionic cathode therein, means for heating said cathode, an anode,and means for delivering to the space between said electrodes caesiumvapor at a pressure sufficiently high both to enable appreciableionization by collision to occur and to materially increase the electronemissivity of the cathode at operating temperatures. V

9. An electron discharge device comprisin an evacuated envelope, ananode constituted of a solid conductor, a tun ten cathode, means forheating said catho e by passage of current, a charge of caesium in saidenvelope and means for maintaining the temperature of said envelope atabout 150 C. during operation.

10. An electric rectifier comprisin an evacuated container, a thermioniccat ode, means for heating said cathode, an anode operable belowelectron-emitting temperatures, a filling of caesium, and means formaintaining said container at an elevated temperature at which adischarge of substantial current value may be maintained therein at acathode temperature which the characteristic emission from said cathodeis inappreciable.

11. An electrical discharge device comprising cooperating electrodes,means for enclosing the space between said electrodes, and a quantity ofcaesium, said device being constructed to operate at temperatures atwhich caesium has vapor pressure of about 0.02 mm. of mercury.

12. The method of operating an electron dischar e device containing acathode which is opera le at an elevated temperature which consists inbringing caesium vapor into concathode at a temperature at which theelectron emission from said cathode is increased by said caesium vapor.

13. The method of operating an electron discharge device com rising acathode adapted to be heated, an an anode, which consists in supplyingcaesium vapor to the space between cathode and anode at a pressure highenough to produce positive ionization by electron impact, and heatingthe cathode below white incandescence to a temperature at which asubstantial electron emission 0ccurs due to said caesium vapor, andproducing an electron discharge while the cathode is at saidtemperature.

14. The method of operating an electron discharge device comprisin acathode adapted to be heated and an ano c, which consists in heating thecathode to a temperature of about 1400 to 1700 K., and increasing theelectron emission from the cathode by sn plying caesium vapor at apressure big enough to permit of suificient ionization by collision tomaterially lower the space charge between the electrodes.

15. The method of operating an electrical discharge device containing acathode operable at an elevated temperature, an anode and a quantity ofcaesium which consists in heatin said device to a temperature of about150 during operation, and applying to said electrodes a voltagesufficient to maintain a discharge which will heat the cathode to anelectron emitting temperature, below white incandescence.

16. The method of operating a thermionic discharge device containin acharge of alkali metal, the vapor of w ich has a lower ionizationpotential than the work function of the cathode of said device, whichconsists in maintaining said device at a temperature above about 65 (1.,operating said cathode at a temperature below about l700 K. andimpressing a voltage between said electrodes capable of producing anionization discharge in the vapor of said alkali metal.

17. An electric discharge device comprisin a cathode and an anode, asealed contamer therefor, and a vapor in said container which has alower ionization otential than the work function of said cat ode andwhich is capable under normal operation of the device of materiallydecreasin the eflective work function of said catho e.

In witness whereof, Gnonen M. J. NIAGKAY has hereunto set his hand this25th day of August, 1926, and Emmsr E. Cmnuron has hereunto set his handthis 13th day of August, 1926 GEORGE M. J. MACKAY. ERNEST E. CHARLTON.

tron emission from said cathode is increased by said caesium vapor.

13. The method of operating an electron discharge device comprising acathode adapted to be heated, an an anode, which consists in supplyingcaesium vapor to the space between cathode and anode at a pressure highenough to produce positive ionization by electron impact, and heatingthe cathode below white incandescence to a temperature at Which asubstantial electron emission oc curs due to said caesium vapor, andproducing an electron discharge while the cathode is at saidtemperature.

14. The method of operating an electron discharge device comprisin acathode adapted to be heated and an arm e, whlch consists in heating thecathode to a temperature of about 1400 to 1700 K., and increasing theelectron emission from the cathode by so plying caesium vapor at apressure big enough to permit of sufficient ionization by collision tomaterially lower the space charge between the electrodes.

15. The method of operating an electrical discharge device containing acathode operable at an elevated temperature, an anode and a quantity ofcaesium which consists in heatin said device to a temperature of about150 during operation, and applying to said electrodes a voltagesuiiicient to maintain a discharge which will heat the cathode to anelectron emitting temperature, below white incandescence.

16. The method of operating a thermionic discharge device containin acharge of alkali metal, the vapor of w ich has a lower ionizationpotential than the work function of the cathode of said device, whichconsists in maintaining said device at a temperature above about 65 (3.,operating said cathode at a temperature below about 1700 K. andimpressing a voltage between said electrodes capable of producing anionization discharge in the vapor of said alkali metal.

17 An electric discharge device comprising a cathode and an anode, asealed container therefor, and a vapor in said container which has alower ionization potential than the work function of said cathode andwhich is capable under normal operation of the device of materiallydecreasin the effective work function of said catho e.

In witness whereof, GEORGE M. J. MAOKAY has hereunto set his hand this25th day of August,1926, and ERNEST E. CHARLTON has hereunto set hishand this 13th day of August, 1926.

GEORGE M. J. MACKAY. ERNEST E. CHARLTON.

CERTIFICATE OF CORRECTION.

Patent No. I, 648, 458.

Granted November '8, 1927, to

GEORGE M. J. MACKAY. ET AL.

it 'is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 7,line 25 claim 2, before the word "pressure" insert the words "thevapor"; and that the said Letters Patent should be read with thiscorrection therein that the same may conform to the record of the casein the Patent Office.

Signedand sealed this 27th day of December, A. D. 1927.

Sea].

M. J. Moore, Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION.

Patent No. 1,648,458. Granted November '8, 1927, to

GEORGE M. J. MACKAY. ET AL.

it is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 7,line 25, ciaim 2, before the word "pressure" insert the words "thevapor"; and that the said Letters Patent should be read with thiscorrection therein that the same may conform to the record of the casein the Patent Office.

Signedand sealed this 27th day of December, A. D. 1927.

M. J. Moore,

Seal. Acting Commissioner of Patents.

